Clozapine is the gold standard treatment for Parkinson's disease (PD) psychosis based on double blinded, placebo controlled trials, and has also been shown to alleviate tremor and dyskinesia. There is accumulating data suggesting that clozapine may be associated with increased frequency of diabetes mellitus (DM) compared to conventional neuroleptic drugs in treating schizophrenia. Forty-four predominantly geriatric parkinsonian subjects on clozapine for psychosis, tremor or dyskinesia, on an average dose of 50.6 mg/d for a mean duration of 41 months were reviewed. The prevalence of DM in this cohort was 18.1% (8/44). This rate was similar to that reported in the aged-matched general population (prevalence = 19.3% for ages > or = 60 years). In this small study, parkinsonian patients on long-term, low dose clozapine were not at increased risk for developing DM. Larger controlled prospective studies are needed to confirm this.